The present invention relates to the general field of brazing together two materials that present different thermomechanical properties.
More precisely, the invention applies to brazing together a metal piece and a piece made of ceramic material, e.g. based on silicon carbide (SiC) and/or carbon.
The piece made of ceramic material may be constituted by solid silicon carbide. It may also be constituted by a thermostructural composite, and in particular by a ceramic matrix composite (CMC) reinforced by silicon carbide or carbon fibers.
Thermostructural composite materials are characterized by mechanical properties that make them suitable for constituting structural parts, while also conserving these mechanical properties at high temperatures. They are constituted by fiber reinforcement densified by a matrix of refractory material that fills in the pores of the fiber reinforcement, at least in part. The choice of materials for the fibers and for the ceramic is typically made amongst carbon and ceramics (i.e. materials that are neither metallic nor organic), and in particular silicon carbide (SiC).
By way of example, the invention can be used for assembling together a piece made of ceramic material with a metal piece made of an alloy of titanium, aluminum, and vanadium (TA6V) or of Inconel 718 (registered trademark), an alloy based on nickel and having the composition NiCr19Fe19Nb5Mo3.
The mechanical properties of pieces made of ceramic material and the fact that these properties are conserved at high temperature make them materials that are particularly suitable for making pieces that are subjected to high levels of thermomechanical stress, in particular in aviation applications (engine parts, fairing elements). When ceramic materials are reinforced by silicon carbide or carbon fibers, they constitute an alternative to metallic materials, presenting numerous advantages, in particular in terms of weight savings and of operational lifetime.
Conventionally, pieces made of ceramic material and metal pieces are assembled together by a mechanical connection of the riveting or bolting type, but such a connection can sometimes be unsuitable for reasons of size, difficulty of implementation, or weight.
Furthermore, known homogeneous assembly methods for use with ceramic materials and involving organic precursors of ceramics are not suitable for heterogeneous assemblies between a ceramic material and a metal.
Known brazing techniques used for making homogeneous connections between two ceramic materials can be difficult to use for heterogeneously brazing a ceramic material on a metal because of the very different thermomechanical and chemical behaviors of ceramic materials and metals.
A metal alloy based on titanium, aluminum, and vanadium presents a coefficient of expansion that is about two to three times greater than that of ceramic materials.
More precisely, the coefficient of expansion of such an alloy at 500° C. is about 10×10−6K−1±15%, while the coefficient for a CMC is about 2.5×10−6K−1 to 4.0×10−6K−1±15%.
Thus, for a 30 millimeter (mm) assembly, an expansion offset of 0.2 mm is observed on cooling the assembly from the solidification temperature of the brazing composition to ambient temperature.
Such an offset leads to high levels of stress appearing in the two pieces, and in particular to compression forces in zones of the brazed joint adjacent to the ceramic, and traction forces in zones adjacent to the metal piece. These stresses can give rise to local deformations that might cause one of the pieces to break or lead to reduced strength of the brazed joint.
Such deformations are irreversible in the metal piece. In the ceramic piece, in particular when made of a CMC, these deformations can lead to brittle type breakage. Such breakage can occur suddenly if the stress is too high. Breakage can also occur by damage building up successively under cyclical stressing.